The present invention relates to a kiln for firing or casting prosthodontic products.
In the manufacture of dental prostheses, false teeth made of ceramic material, supported on a core or support constituted by a titanium casting, are becoming increasingly widespread. The stable coupling between the ceramic material and the titanium support is achieved by firing the ceramic material on the metal support. In order to avoid the formation of oxides on the surface of the metal support, which would compromise the metallic structure and thus the adhesion and anchorage of the ceramic material, firing must occur in an inert atmosphere.
The kilns that have been used so far allow firing in an inert atmosphere, but the operator, after removing the part from the muffle, must wait for a long time (a few hours) before he can continue with the necessary work for finishing the prosthesis, since it is necessary to wait for the complete cooling of the prosthesis in the closed kiln; the productivity of a firing operation is therefore very low.
Italian patent application VR93A000012, filed Feb. 18, 1993 in the name of the same Applicant, discloses a process and an apparatus for casting in an inert atmosphere which uses a crucible placed adjacent to a casting cylinder which contains refractory material with an impression: the crucible and the cylinder are located within a sealed casting chamber. The casting chamber can be tilted to cast the molten metal into the crucible by gravity. The ambient air contained by the crucible section and by the cylinder section is aspirated by means of a pneumatic circuit connected to a vacuum pump. After producing a certain degree of vacuum, the aspiration circuit is disconnected and inert gas is introduced from the crucible section; said gas passes through the chamber that contains the crucible and the casting chamber and the related impressions, performing an effective flushing action. After casting, inert gas at a relatively high pressure is introduced in the chamber that contains the crucible in order to inject, by pressure, the molten metal into the casting cylinder; the aspiration circuit is then disconnected from the casting chamber section as well, the unit is opened and the casting cylinder is removed from the muffle.
Such a solution according to the above mentioned patent application, while being a considerable improvement over the prior state of the art, is in practice affected by some limitations that sometimes negatively affect the quality of the products, which is not always uniform. First of all, lack of control over pressure, which does not remain constant during casting, can cause non-uniform castings and defects in the form of bubbles or porosities. The continuous operation of the aspiration circuit in the casting cylinder section can cause a relatively high pressure difference and thus increase the casting speed, inducing vortical motions in the liquid and thus bubbles inside the cast. Furthermore, the refractory material that constitutes the impression is porous and thus tends to absorb and retain oxygen particles even after vacuum has been produced, releasing them later during casting, thereby reducing the quality of the cast.